Techniques for improving SNR in a FMCW LiDAR system using a coherent receiver

ABSTRACT

The LiDAR system includes a coherent receiver disposed in a reference path. The coherent receiver includes a 90° optical hybrid to receive a portion of an optical beam along the reference path and a local oscillator (LO) signal to generate multiple output signals. The coherent receiver includes a first photodetector to receive a first and a second output signal to generate a first mixed signal, and a second photodetector to receive a third and a fourth output signal to generate a second mixed signal. The LiDAR system further includes a processor to combine the first mixed signal and the second mixed signal to generate a combined reference signal to suppress a negative image of a reference beat frequency signal to estimate a phase noise of the optical source to determine range and velocity information of the target.

TECHNICAL FIELD

The present disclosure relates generally to light detection and ranging(LiDAR) systems, and more particularly to a FMCW LiDAR system with acoherent receiver.

BACKGROUND

Frequency-Modulated Continuous-Wave (FMCW) LiDAR systems may use areference optical path with a delay of known length to create areference beat frequency. A linear representation of the phase noise ofan optical source may be extracted from the reference beat frequency inorder to remove the peak spreading effects of the phase noise for atarget. With a single or balanced coherent detector in the referenceoptical path, the phase noise from the positive image and the negativeimage of the reference beat signal in the frequency domain may overlap,which may degrade the quality of the linear phase noise estimation andresult in a lowered signal to noise ratio (SNR) for the target.

SUMMARY

The present disclosure describes various examples of coherent referencereceivers in LiDAR systems, e.g., in a FMCW LiDAR system.

In some examples, disclosed herein is a FMCW LiDAR system using acoherent reference receiver for improving a target SNR. The coherentreference receiver in a reference delay path may include a 90° opticalhybrid receiver feeding the full complex beat signal into the signalprocessing chain. Combining the outputs of the 90° optical hybridreceiver results in suppression of the negative frequency image. Somesignal processing schemes are provided to correct for the imperfectionsof the 90° optical hybrid receiver, and to account for the effects ofup/down chirped signals from an optical source. By this method, thetarget SNR is improved with image suppression for low reference beatfrequencies.

In some examples, a LiDAR system is disclosed herein. The LiDAR systemincludes an optical source to emit an optical beam along a target pathtowards a target and a reference path. The LiDAR system includes acoherent receiver disposed in the reference path. The coherent receiverincludes a 90° optical hybrid to receive a portion of the optical beamalong the reference path and a local oscillator (LO) signal to generatea first, a second, a third and a fourth output signal. The coherentreceiver includes a first photodetector to receive the first and thesecond output signal to generate a first mixed signal. The coherentreceiver further includes a second photodetector to receive the thirdand the fourth output signal to generate a second mixed signal. Thecoherent receiver is disposed to mix the portion of the optical beamwith the LO signal. The LiDAR system further includes a processor tocombine the first mixed signal and the second mixed signal to generate acombined reference signal to suppress a negative image of a referencebeat frequency signal produced by the optical beam and the LO signal toestimate a phase noise of the optical source to determine range andvelocity information of the target.

In some examples, a method of light detection and ranging is disclosedherein. The method includes emitting an optical beam by an opticalsource along a target path towards a target and a reference path. Themethod includes receiving and mixing a portion of the optical beam and alocal oscillator (LO) signal by a coherent receiver disposed in thereference path. The method includes receiving the portion of the opticalbeam and the LO signal by a 90° optical hybrid to generate a first, asecond, a third and a fourth output signal. The method includesreceiving the first and the second output signal by a firstphotodetector to generate a first mixed signal. The method includesreceiving the third and the fourth output signal by a secondphotodetector to generate a second mixed signal. The method furtherincludes combining, by a processor, the first mixed signal and thesecond mixed signal to generate a combined reference signal to suppressa negative image of a reference beat frequency signal produced by theoptical beam and the LO signal to estimate a phase noise of the opticalsource to determine range and velocity information of the target.

It should be appreciated that, although one or more embodiments in thepresent disclosure depict the use of point clouds, embodiments of thepresent invention are not limited as such and may include, but are notlimited to, the use of point sets and the like.

These and other aspects of the present disclosure will be apparent froma reading of the following detailed description together with theaccompanying figures, which are briefly described below. The presentdisclosure includes any combination of two, three, four or more featuresor elements set forth in this disclosure, regardless of whether suchfeatures or elements are expressly combined or otherwise recited in aspecific example implementation described herein. This disclosure isintended to be read holistically such that any separable features orelements of the disclosure, in any of its aspects and examples, shouldbe viewed as combinable unless the context of the disclosure clearlydictates otherwise.

It will therefore be appreciated that this Summary is provided merelyfor purposes of summarizing some examples so as to provide a basicunderstanding of some aspects of the disclosure without limiting ornarrowing the scope or spirit of the disclosure in any way. Otherexamples, aspects, and advantages will become apparent from thefollowing detailed description taken in conjunction with theaccompanying figures which illustrate the principles of the describedexamples.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of various examples, reference is nowmade to the following detailed description taken in connection with theaccompanying drawings in which like identifiers correspond to likeelements:

FIG. 1A is a block diagram illustrating an example LiDAR systemaccording to embodiments of the present disclosure.

FIG. 1B is a block diagram illustrating an example of a coherentreceiver in a LiDAR system according to embodiments of the presentdisclosure.

FIG. 2 is a time-frequency diagram illustrating an example of FMCW LIDARwaveforms according to embodiments of the present disclosure.

FIG. 3 is a block diagram illustrating an example of a LiDAR systemusing a coherent reference receiver according to embodiments of thepresent disclosure.

FIG. 4A is a block diagram illustrating an example of a 90° opticalhybrid according to embodiments of the present disclosure.

FIGS. 4B-4D are illustrating an example of image suppressing using an90° optical hybrid according to embodiments of the present disclosure.

FIG. 5A is a diagram illustrating an example of a chirp signal with upand down chirping according to embodiments of the present disclosure.

FIGS. 5B-5C are diagrams illustrating an example of a combined referencesignal without or with applying a polarity correction respectivelyaccording to embodiments of the present disclosure.

FIG. 6 is a flow diagram illustrating an example of applying a polaritycorrection according to embodiments of the present disclosure.

FIGS. 7A-7B are diagrams illustrating an example of correcting forimperfections of a 90° optical hybrid receiver, according to embodimentsof the present disclosure.

FIG. 8 is a flow diagram illustrating an example of a process ofcorrecting for imperfections of a 90° optical hybrid receiver accordingto embodiments of the present disclosure.

FIG. 9 is a flow diagram illustrating an example of a method of using acoherent reference receiver in a LiDAR system according to embodimentsof the present disclosure.

DETAILED DESCRIPTION

Various embodiments and aspects of the disclosures will be describedwith reference to details discussed below, and the accompanying drawingswill illustrate the various embodiments. The following description anddrawings are illustrative of the disclosure and are not to be construedas limiting the disclosure. Numerous specific details are described toprovide a thorough understanding of various embodiments of the presentdisclosure. However, in certain instances, well-known or conventionaldetails are not described in order to provide a concise discussion ofembodiments of the present disclosures.

The described LiDAR systems herein may be implemented in any sensingmarket, such as, but not limited to, transportation, manufacturing,metrology, medical, virtual reality, augmented reality, and securitysystems. According to some embodiments, the described LiDAR system maybe implemented as part of a front-end of frequency modulatedcontinuous-wave (FMCW) device that assists with spatial awareness forautomated driver assist systems, or self-driving vehicles.

FIG. 1A illustrates a LiDAR system 100 according to exampleimplementations of the present disclosure. The LiDAR system 100 includesone or more of each of a number of components, but may include fewer oradditional components than shown in FIG. 1A. According to someembodiments, one or more of the components described herein with respectto LiDAR system 100 can be implemented on a photonics chip. The opticalcircuits 101 may include a combination of active optical components andpassive optical components. Active optical components may generate,amplify, and/or detect optical signals and the like. In some examples,the active optical component includes optical beams at differentwavelengths, and includes one or more optical amplifiers, one or moreoptical detectors, or the like.

Free space optics 115 may include one or more optical waveguides tocarry optical signals, and route and manipulate optical signals toappropriate input/output ports of the active optical circuit. The freespace optics 115 may also include one or more optical components such astaps, wavelength division multiplexers (WDM), splitters/combiners,polarization beam splitters (PBS), collimators, couplers or the like. Insome examples, the free space optics 115 may include components totransform the polarization state and direct received polarized light tooptical detectors using a PBS, for example. The free space optics 115may further include a diffractive element to deflect optical beamshaving different frequencies at different angles.

In some examples, the LiDAR system 100 includes an optical scanner 102that includes one or more scanning mirrors that are rotatable along anaxis (e.g., a slow-moving-axis) that is orthogonal or substantiallyorthogonal to the fast-moving-axis of the diffractive element to steeroptical signals to scan a target environment according to a scanningpattern. For instance, the scanning mirrors may be rotatable by one ormore galvanometers. Objects in the target environment may scatter anincident light into a return optical beam or a target return signal. Theoptical scanner 102 also collects the return optical beam or the targetreturn signal, which may be returned to the passive optical circuitcomponent of the optical circuits 101. For example, the return opticalbeam may be directed to an optical detector by a polarization beamsplitter. In addition to the mirrors and galvanometers, the opticalscanner 102 may include components such as a quarter-wave plate, lens,anti-reflective coating window or the like.

To control and support the optical circuits 101 and optical scanner 102,the LiDAR system 100 includes LiDAR control systems 110. The LiDARcontrol systems 110 may include a processing device for the LiDAR system100. In some examples, the processing device may be one or moregeneral-purpose processing devices such as a microprocessor, centralprocessing unit, or the like. More particularly, the processing devicemay be complex instruction set computing (CISC) microprocessor, reducedinstruction set computer (RISC) microprocessor, very long instructionword (VLIW) microprocessor, or processor implementing other instructionsets, or processors implementing a combination of instruction sets. Theprocessing device may also be one or more special-purpose processingdevices such as an application specific integrated circuit (ASIC), afield programmable gate array (FPGA), a digital signal processor (DSP),network processor, or the like.

In some examples, the LiDAR control systems 110 may include a signalprocessing unit 112 such as a digital signal processor (DSP). The LiDARcontrol systems 110 are configured to output digital control signals tocontrol optical drivers 103. In some examples, the digital controlsignals may be converted to analog signals through signal conversionunit 106. For example, the signal conversion unit 106 may include adigital-to-analog converter. The optical drivers 103 may then providedrive signals to active optical components of optical circuits 101 todrive optical sources such as lasers and amplifiers. In some examples,several optical drivers 103 and signal conversion units 106 may beprovided to drive multiple optical sources.

The LiDAR control systems 110 are also configured to output digitalcontrol signals for the optical scanner 102. A motion control system 105may control the galvanometers of the optical scanner 102 based oncontrol signals received from the LIDAR control systems 110. Forexample, a digital-to-analog converter may convert coordinate routinginformation from the LiDAR control systems 110 to signals interpretableby the galvanometers in the optical scanner 102. In some examples, amotion control system 105 may also return information to the LiDARcontrol systems 110 about the position or operation of components of theoptical scanner 102. For example, an analog-to-digital converter may inturn convert information about the galvanometers' position to a signalinterpretable by the LIDAR control systems 110.

The LiDAR control systems 110 are further configured to analyze incomingdigital signals. In this regard, the LiDAR system 100 includes opticalreceivers 104 to measure one or more beams received by optical circuits101. For example, a reference beam receiver may measure the amplitude ofa reference beam from the active optical component, and ananalog-to-digital converter converts signals from the reference receiverto signals interpretable by the LiDAR control systems 110. Targetreceivers measure the optical signal that carries information about therange and velocity of a target in the form of a beat frequency,modulated optical signal. The reflected beam may be mixed with a secondsignal from a local oscillator. The optical receivers 104 may include ahigh-speed analog-to-digital converter to convert signals from thetarget receiver to signals interpretable by the LiDAR control systems110. In some examples, the signals from the optical receivers 104 may besubject to signal conditioning by signal conditioning unit 107 prior toreceipt by the LiDAR control systems 110. For example, the signals fromthe optical receivers 104 may be provided to an operational amplifierfor amplification of the received signals and the amplified signals maybe provided to the LIDAR control systems 110.

In some applications, the LiDAR system 100 may additionally include oneor more imaging devices 108 configured to capture images of theenvironment, a global positioning system 109 configured to provide ageographic location of the system, or other sensor inputs. The LiDARsystem 100 may also include an image processing system 114. The imageprocessing system 114 can be configured to receive the images andgeographic location, and send the images and location or informationrelated thereto to the LiDAR control systems 110 or other systemsconnected to the LIDAR system 100.

In operation according to some examples, the LiDAR system 100 isconfigured to use nondegenerate optical sources to simultaneouslymeasure range and velocity across two dimensions. This capability allowsfor real-time, long range measurements of range, velocity, azimuth, andelevation of the surrounding environment.

In some examples, the scanning process begins with the optical drivers103 and LiDAR control systems 110. The LiDAR control systems 110instruct the optical drivers 103 to independently modulate one or moreoptical beams, and these modulated signals propagate through the passiveoptical circuit to the collimator. The collimator directs the light atthe optical scanning system that scans the environment over apreprogrammed pattern defined by the motion control system 105. Theoptical circuits 101 may also include a polarization wave plate (PWP) totransform the polarization of the light as it leaves the opticalcircuits 101. In some examples, the polarization wave plate may be aquarter-wave plate or a half-wave plate. A portion of the polarizedlight may also be reflected back to the optical circuits 101. Forexample, lensing or collimating systems used in LIDAR system 100 mayhave natural reflective properties or a reflective coating to reflect aportion of the light back to the optical circuits 101.

Optical signals reflected back from the environment pass through theoptical circuits 101 to the receivers. Because the polarization of thelight has been transformed, it may be reflected by a polarization beamsplitter along with the portion of polarized light that was reflectedback to the optical circuits 101. Accordingly, rather than returning tothe same fiber or waveguide as an optical source, the reflected light isreflected to separate optical receivers. These signals interfere withone another and generate a combined signal. Each beam signal thatreturns from the target produces a time-shifted waveform. The temporalphase difference between the two waveforms generates a beat frequencymeasured on the optical receivers (photodetectors). The combined signalcan then be reflected to the optical receivers 104.

The analog signals from the optical receivers 104 are converted todigital signals using ADCs. The digital signals are then sent to theLiDAR control systems 110. A signal processing unit 112 may then receivethe digital signals and interpret them. In some embodiments, the signalprocessing unit 112 also receives position data from the motion controlsystem 105 and galvanometers (not shown) as well as image data from theimage processing system 114. The signal processing unit 112 can thengenerate a 3D point cloud with information about range and velocity ofpoints in the environment as the optical scanner 102 scans additionalpoints. The signal processing unit 112 can also overlay a 3D point clouddata with the image data to determine velocity and distance of objectsin the surrounding area. The system also processes the satellite-basednavigation location data to provide a precise global location.

FIG. 1B is a block diagram illustrating an example of a coherentreceiver 160 in the LiDAR system 100 according to embodiments of thepresent disclosure. The LIDAR system 100 further includes a referencearm to generate one or more digitally sampled reference signals. Forexample, the LiDAR system 100 includes an optical source to emit anoptical beam along a target path towards a target and a reference pathwith a delay of known length. The LiDAR system 100 includes a coherentreference receiver in the reference path for improving the SNR for thetarget. The coherent receiver 160 includes a 90° optical hybrid 130 toreceive a portion of the optical beam along the reference path and a LOsignal in the reference path to generate multiple output signals. Thecoherent receiver 160 further includes one or more photodetectors (e.g.,131, 132) to receive the output signals of the 90° optical hybrid 130and to generate one or more mixed signals. The one or more mixed signalsare used to estimate the phase noise of the optical source in thetransmitted signals.

As depicted in FIG. 1B, the signal processing unit 112 of the LiDARsystem 100 includes a digital signal processing module 117, whichincludes a combination module 121 and a phase noise estimation module124. The combination module 121 is configured to combine the one or moremixed signals to generate a combined reference signal to suppress anegative image of a reference beat frequency signal produced by theoptical beam and the LO signal. The phase noise estimation module 124 isconfigured to estimate a phase noise of the optical source to determinerange and velocity information of the target. The combination module 121may include a polarity correction module 122 to account for the effectsof up/down chirped signals from the optical source. The combinationmodule 121 may further include an image rejection ratio module 123 tocorrect for the imperfections of the 90° optical hybrid. In this way,the negative image of the reference beat frequency signal is suppressed.Thus, the SNR for the target is improved. Therefore, the accuracy of therange and velocity information of the target is improved.

FIG. 2 is a time-frequency diagram 200 of an FMCW scanning signal 201that can be used by a LiDAR system, such as system 100, to scan a targetenvironment according to some embodiments. In one example, the scanningsignal waveform 201, labeled as fFM(t), is a sawtooth waveform (sawtooth“chirp”) with a chirp bandwidth ΔfC and a chirp period TC. The slope ofthe sawtooth is given as k=(ΔfC/TC). FIG. 2 also depicts target returnsignal 202 according to some embodiments. Target return signal 202,labeled as fFM(t−Δt), is a time-delayed version of the scanning signal201, where Δt is the round trip time to and from a target illuminated byscanning signal 201. The round trip time is given as Δt=2R/v, where R isthe target range and v is the velocity of the optical beam, which is thespeed of light c. The target range, R, can therefore be calculated asR=c(Δt/2). When the return signal 202 is optically mixed with thescanning signal, a range dependent difference frequency (“beatfrequency”) ΔfR(t) is generated. The beat frequency ΔfR(t) is linearlyrelated to the time delay Δt by the slope of the sawtooth k. That is,ΔfR(t)=kΔt. Since the target range R is proportional to Δt, the targetrange R can be calculated as R=(c/2)(ΔfR(t)/k). That is, the range R islinearly related to the beat frequency ΔfR(t). The beat frequency ΔfR(t)can be generated, for example, as an analog signal in optical receivers104 of system 100. The beat frequency can then be digitized by ananalog-to-digital converter (ADC), for example, in a signal conditioningunit such as signal conditioning unit 107 in LIDAR system 100. Thedigitized beat frequency signal can then be digitally processed, forexample, in a signal processing unit, such as signal processing unit 112in system 100. It should be noted that the target return signal 202will, in general, also include a frequency offset (Doppler shift) if thetarget has a velocity relative to the LIDAR system 100. The Dopplershift can be determined separately, and used to correct the frequency ofthe return signal, so the Doppler shift is not shown in FIG. 2 forsimplicity and ease of explanation. It should also be noted that thesampling frequency of the ADC will determine the highest beat frequencythat can be processed by the system without aliasing. In general, thehighest frequency that can be processed is one-half of the samplingfrequency (i.e., the “Nyquist limit”). In one example, and withoutlimitation, if the sampling frequency of the ADC is 1 gigahertz, thenthe highest beat frequency that can be processed without aliasing(ΔfRmax) is 500 megahertz. This limit in turn determines the maximumrange of the system as Rmax=(c/2)(ΔfRmax/k) which can be adjusted bychanging the chirp slope k. In one example, while the data samples fromthe ADC may be continuous, the subsequent digital processing describedbelow may be partitioned into “time segments” that can be associatedwith some periodicity in the LIDAR system 100. In one example, andwithout limitation, a time segment might correspond to a predeterminednumber of chirp periods T, or a number of full rotations in azimuth bythe optical scanner.

FIG. 3 is a block diagram 300 illustrating an example of the LiDARsystem 100 using the coherent receiver 160, according to embodiments ofthe present disclosure. For instance, the system 100 includes theoptical source 301, such as a FMCW laser source. The target arm 305includes a number of optical components (e.g. lenses, filters, and thelike) through which the optical beam 303, which includes the scanningsignal, passes on its way to a target 307. The return signal 309 may bereflected from the target 307 and directed to a photo detector 311(e.g., included in the optical receivers 104 in FIG. 1A). In someembodiments, a local oscillator (LO) signal 313, which is a portion ofthe optical beam 303, is directed to the photo detector 311 to mix withthe return signal 309. From the photo detector 311, a digitally sampledtarget signal 316 then passes to a target ADC 315, and then to the DSP117.

As depicted in FIG. 3 , the LIDAR system 100 includes a reference arm347 to generate one or more digitally sampled reference signals 326, 328that can be used to estimate the phase noise of the optical source 301in the transmitted signals. In this fashion, reference arm 347 createsone or more digitally sampled reference signals 326, 328 correspondingto the target (e.g., target 307) at a known delay with the phase noisesimilar to that on the received signal from the target 307. The one ormore digitally sampled reference signals 326, 328 may be used toestimate the phase noise of the optical source 301 for subsequentcorrection.

For instance, the reference arm 347 receives a portion 319 of theoptical beam 303, which may be provided to the coherent receiver 160directly, and also after passing through a delay device 323 having aknown length and/or delay. According to some embodiments, the signalportion 319 is received by the coherent receiver 160 in the referencearm 347 as the scanning signal of the optical beam 303 is transmittedcontemporaneously through the optical components of the target arm 305.According to some embodiments, the signal portion 319 is received by thecoherent receiver 160 after the scanning signal of the optical beam 303is transmitted through the optical components of the target arm 305.According to some embodiments, the delay device 323 may be a fiber delaydevice, etc. In one embodiment, the delay device 323 may include a fibercoil with a known length that may create a virtual target (e.g., fibertarget) at a known distance.

In some scenarios, the virtual target's distance may be pre-determined.An optical signal 339 at the output of the reference delay 323 may havethe same characteristics as the target return signal 202 depicted inFIG. 2 . According to some embodiments, in a manner similar to thatdescribed in FIG. 2 , virtual targets described herein may produce theoptical signal 339 that is a time-delayed version of the optical beam303.

Referring to FIG. 3 , the coherent receiver 160 may receive the opticalsignal 339 and a reference LO signal 333. As an example, the referencearm 347 may also include a reference LO generator 335 to generate thereference LO signal 333, which is a portion of the optical beam 303. Theoptical signal 339 is the portion 319 of the optical beam 303 after thereference delay 323 along the reference path. When the optical signal339 is optically mixed with the reference LO signal 333, a referencebeat frequency is generated. The coherent receiver 160 includes the 90°optical hybrid 130, which has two inputs to receive the optical signal339 and the reference LO signal 333 in the reference path. The 90°optical hybrid 130 further has four outputs 341A, 341B, 341C, 341D,which mix the two input optical signals in 90° phase intervals from eachother (which will be discussed in detail below).

The coherent receiver 160 may include one or more photo detectors 131,132 to output the reference arm signals 326, 328. For example, the photodetectors 131 may receive the output signals 341A, 341B and generate themixed signal 326, and the photo detectors 132 may receive the outputsignals 341C, 341D and generate the mixed signal 328.

The mixed signals 326, 328 may pass to reference ADCs 325, 327, and thento the DSP 117. The mixed signals 326, 328 may be combined, at the DSP117, to generate a combined reference signal to suppress a negativeimage of the reference beat frequency signal produced by the opticalsignal 339 and the reference LO signal 333. Combining the outputs of the90° optical hybrid receiver in the manner described by embodiments ofthe present disclosure results in the suppression of the negativefrequency image of the reference beat frequency signal. In this way, thetarget SNR is improved with the image suppression for low reference beatfrequencies. Therefore, the accuracy of determining the range andvelocity information of the target is improved.

As an example, the mixed signals 326, 328 form the photodetector 131,132 may be used to feedback to the driver of the optical source 301 tolinearize or provide feedback control of the scan signal. The range andvelocity information of the target may be determined according to thetransmission and receipt of various signals, including the digitallysampled target signal 316 and the combined reference signal based on thedigitally sampled reference signals 326, 328. As an example, a pointcloud 329 may be produced to determine range and velocity information ofthe target. Some signal processing schemes may be used to correct forthe imperfections of the 90° optical hybrid receiver, and to account forthe effects of up/down chirped signals from an optical source.

FIG. 4A is a block diagram illustrating an example of the 90° opticalhybrid 130 according to embodiments of the present disclosure. FIGS.4B-4D are illustrating an example of image suppressing using the 90°optical hybrid 130 according to embodiments of the present disclosure.Referring to FIG. 4A and FIGS. 4B-4D, utilizing the coherent receiver160 with the 90° optical hybrid 130, both the real and complex portionsof the reference beat frequency may be recovered and processed withdigital signal processing, e.g., at the DSP 117 in the signal processingunit 112 in FIG. 1A.

As depicted in FIG. 4A, the 90° optical hybrid 130 has two inputs toreceive the optical signal 339 and the reference LO signal 333 in thereference path. The optical signal 339 and the reference LO signal 333may be expressed as below:Signal=S=E _(s)e^(jω) ^(s) ^(t)  (1)LO=L=E _(LO)e^(jω) ^(LO) ^(t)  (2)where S represents the optical signal 339, and L represents thereference LO signal 333.

Referring to FIGS. 4A-4D, the 90° optical hybrid 130 may include lineardividers and combiners interconnected in such a way that four differentvectorial additions of the reference LO signal 333 and the opticalsignal 339 are obtained. As an example, the 90° optical hybrid 130 maymix the incoming optical signal 339 with the four quadratural statesassociated with the reference LO signal 333 in the complex-field space.The 90° optical hybrid 130 further has four primary outputs 341A, 341B,341C, 341D, which mix the two input optical signals 333, 339 in 90°phase intervals from each other.

For example, the four output signals 341A, 341B, 341C, 341D may bedelivered to photodetectors 131, 132. The photodetector 131 may receivethe output signal 341A and the output signal 341B to generate a mixedsignal 326. The photodetector 132 may receive the output signal 341C andthe output signal 341D to generate a mixed signal 328. The coherentreceiver 160 may have two output signals 326, 328. The output mixedsignal 326 may be called I, which may be the real numbered portion ofthe reference beat frequency. The output mixed signal 328 may be calledQ, which may be the imaginary numbered portion of the reference beatfrequency.

Referring to FIGS. 4B-4D, as an example, the process of suppressing thenegative frequency image of the reference beat frequency signal may beexpressed as below. If each of the mixed signals (e.g., I, Q) is asimple sine tone at the reference beat frequency f1, each of the mixedsignals may appear as both a peak (e.g., 401 a, 402 a) at the positivereference beat frequency f1 and a peak (e.g., 401 b, 402 b) at thenegative reference beat frequency −f1, in the frequency domain. The realportion I of the reference beat signal, may be expressed as:

$\begin{matrix}{I = {{I_{p} - I_{n}} = {{R\left( {{\frac{1}{4}{❘{S + L}❘}^{2}} - {\frac{1}{4}{❘{S - L}❘}^{2}}} \right)} = {{R\frac{1}{4}\left( {{\left( {S + L} \right)\left( {S^{*} + L^{*}} \right)} - {\left( {S - L} \right)\left( {S^{*} - L^{*}} \right)}} \right)} = {{R\frac{1}{4}\left( {{❘S❘}^{2} + {❘L❘}^{2} + {SL}^{*} + {S^{*}L} - {❘S❘}^{2} - {❘L❘}^{2} + {SL}^{*} + {S^{*}L}} \right)} = {R\frac{1}{2}\left( {{SL}^{*} + {S^{*}L}} \right)}}}}}} & (3)\end{matrix}$where R is the photodetector responsivity.

The complex portion Q of the reference beat signal may be expressed as:

$\begin{matrix}{Q = {{Q_{p} - Q_{n}} = {{R\left( {{\frac{1}{4}{❘{S + {jL}}❘}^{2}} - {\frac{1}{4}{❘{S - {jL}}❘}^{2}}} \right)} = {{R\frac{1}{4}\left( {{\left( {S + {jL}} \right)\left( {S^{*} - {jL}^{*}} \right)} - {\left( {S - {jL}} \right)\left( {S^{*} + {jL}^{*}} \right)}} \right)} = {{R\frac{1}{4}\left( {{❘S❘}^{2} + {❘L❘}^{2} + {{jS}^{*}L} - {jSL}^{*} - {❘S❘}^{2} - {❘L❘}^{2} + {{jS}^{*}L} - {jSL}^{*}} \right)} = {R\frac{j}{2}\left( {{S^{*}L} - {SL}^{*}} \right)}}}}}} & (4)\end{matrix}$where j is the imaginary number j=sqrt(−1), and R is the photodetectorresponsivity.

The output signal 326 and the output signal 328 are 90° from each other,and are combined to return a single combined reference signal 403, whichis a complex exponential with a phase being the frequency differencebetween the two input signals 333, 339. The combined reference signal403 is generated to suppress the negative image of the reference beatfrequency signal. For example, the combined reference signal 403 may beexpressed as:

$\begin{matrix}{{I + {jQ}} = {{R\frac{1}{2}\left( {\left( {{SL}^{*} + {S^{*}L}} \right) - \left( {{S^{*}L} - {SL}^{*}} \right)} \right)} = {{RSL}^{*} = {{RE}_{x}E_{LO}e^{{j({\omega_{s} - \omega_{LO}})}t}}}}} & (5)\end{matrix}$

By combining the output signal 326 and the output signal 328 with imagesuppressing, the single combined reference signal appears as a singlepeak 403 a, while the negative peak 403 b being suppressed, e.g., to beclose to zero. With the IQ based coherent receiver 160, the negativeimage of the reference beat frequency is suppressed. Thus, the SNR forthe target is improved. Therefore, the accuracy of determining the rangeand velocity of the target is increased.

In some other embodiments, if balanced or differential detection is notpreferred to save cost and reduce complexity, utilizing all 4 outputsignals from the 90° optical hybrid 130 may not be necessary forsuppressing the negative image of the reference beat frequency signal.Single ended detection of the I and Q signals may be used to produce animage rejection, as long as the signals selected for the I and thesignal selected for the Q are still 90° apart from each other. As anexample, I=Ip and Q=Qp, or I=I p and Q=Qn.

Referring back to FIG. 4A, when single ended detection of the I and Qsignals is used, for example, the photodetector 131 may receive theoutput signal 341A. The photodetector 132 may receive the output signal341C. The output mixed signal 326, which may be called I, is equivalentto 341A (e.g., Ip). The output mixed signal 328, which may be called Q,is equivalent to 341C (e.g., Qp).

In this example, the real portion I of the reference beat frequency, maybe expressed as:I=I _(p) =R(¼|S+L| ²)=R¼((|S| ² +|L| ² +SL*+S*L)  (6)

The complex portion Q of the reference beat frequency may be expressedas:Q=Qp=R(¼|S+jL| ²)=R¼((|S| ² +|L| ² +jS*L−jSL*)  (7)

For example, the combined reference signal 403 may be expressed as:

$\begin{matrix}{{I + {jQ}} = {{R\frac{1}{4}\left( {{❘S❘}^{2} + {❘L❘}^{2} + {SL}^{*} + {S^{*}L} + {j{❘S❘}^{2}} + {j{❘L❘}^{2}} - {S^{*}L} + {SL}^{*}} \right)} = {R\frac{1}{4}\left( {{{\left( {1 + j} \right)\left( {{❘S❘}^{2} + {❘L❘}^{2}} \right)} + {R\frac{1}{2}{SL}^{*}}} = {{DC} + {R\frac{1}{2}E_{x}E_{LO}e^{{j({\omega_{s} - \omega_{LO}})}t}}}} \right.}}} & (8)\end{matrix}$

By using the single ended detection of the I and Q signals, the totalsignal power may be reduced, e.g., by 3 dB. There may be a DC termremaining as well.

FIG. 5A is a diagram illustrating an example of a chirp signal with upand down chirping according to embodiments of the present disclosure.Referring to FIG. 5A, in some embodiments, the LiDAR system 100 mayutilize an up and down sweep of frequency. For example, in anup-sweeping (e.g., up-chirping), the frequency of the scanning signalmay be increasing over time; in a down-sweeping (e.g., down-chirping),the frequency of the scanning signal may be decreasing over time. Thus,the frequency difference 530 between the delay path signal (e.g., theoptical signal 339) and the non-delay path signal (e.g., the referenceLO signal 333) may flip in sign depending on the sweep direction (e.g.,chirp direction). For example, during the up sweeping period, the signof the frequency difference between the delay path optical signal 339and the reference LO signal 333 may be negative; during the downsweeping period, the sign of the frequency difference 530 between thedelay path optical signal 339 and the reference LO signal may bepositive. The flip of sign may switch the suppressed image from thenegative axis to the positive axis unless it is corrected for.

FIGS. 5B-5C are diagrams illustrating an example of a combined referencesignal without or with applying a polarity correction respectivelyaccording to embodiments of the present disclosure. As illustrated inFIG. 5B, without applying the polarity correction, because thesuppressed image is switched from the negative axis to the positiveaxis, there are two peaks 501 a, 501 b in the combined reference signalcovering multiple up/down periods, the negative image of the referencebeat signal is not consistently suppressed.

If the positive image (or the negative image) is to be preservedconsistently, the polarity correction is applied to the combination ofthe signal I and the signal Q depending on the sweep direction (e.g.,chirp direction). The signal portion Q may have the alternated positiveand negative sign depending on the direction of the frequency sweeping(e.g., chirping). As an example, a ±1 square wave, which has thealternated positive and negative sign, with a period equal to theup/down sweeping period, and aligned with the start of the up/downsweeping may be used to correct the polarity. When the ±1 square wave isused, the polarity of the frequency difference is corrected. Asillustrated in FIG. 5C, there is only one peak 503 a in the combinedreference signal, thus, the negative image of the reference beat signalis suppressed and not visible.

FIG. 6 is a flow diagram illustrating an example of a method 600 ofapplying a polarity correction according to embodiments of the presentdisclosure. Method 600 may be performed by processing logic that maycomprise hardware (e.g., circuitry, dedicated logic, programmable logic,a processor, a processing device, a central processing unit (CPU), asystem-on-chip (SoC), etc.), software (e.g., instructionsrunning/executing on a processing device), firmware (e.g., microcode),or a combination thereof. For example, the method 600 may be performedby a processor, e.g., a signal processing unit 112 of a LiDAR system, asillustrated in FIG. 1A-FIG. 1B. By this method, when combining the mixedsignals 326, 328 (e.g., I, Q) from the coherent receiver 160, a sign ofthe mixed signal 328 (e.g., Q) is determined depending on a direction offrequency sweeping.

At block 602, the mixed signals 326, 328 (e.g., I, Q) are received fromthe photodetectors 131, 132 of the coherent receiver 160. As illustratedin FIG. 1B, a polarity correction module 122 may receive the mixedsignals 326, 328 (e.g., I, Q) from the photodetectors 131, 132 of thecoherent receiver 160.

At block 604, the polarity correction is applied.

At block 605, a sign associated with an up-sweeping scan is determined,and a sign associated with a down-sweeping scan is determined. The signassociated with the down-sweeping scan is an opposite sign from the signassociated with the up-sweeping scan. At block 606, a direction of thefrequency sweep is determined. For example, a square wave is to beapplied to change the sign of the mixed signal 328 (e.g., Q).

At block 612, for the up-sweeping scan, a square function with the signassociated with the up-sweeping scan is applied to the mixed signal Q.For example, when the direction is an up-sweeping direction, the sign isdetermined to be positive.

At block 614, for the down-sweeping scan, the square function with thesign associated with the down-sweeping scan is applied to the mixedsignal 328 (e.g., Q. For example, when the direction is a down-sweepingdirection, the sign is determined to be negative.

At block 620, the mixed signal 326 (e.g., I) is combined with the mixedsignal 328 (e.g., Q) in the positive sign in up-sweeping or the negativesign in down-sweeping, in order to generate the combined referencesignal (e.g., I+/−jQ).

At block 624, the phase noise of the optical source may be estimated bycomparing the target return signal. (e.g., 309) with the combinedreference signal (e.g., 403).

FIGS. 7A-7B are diagrams illustrating an example of correcting forimperfections of the 90° optical hybrid 130, according to embodiments ofthe present disclosure. There may be imperfections in the optical hybridangle or amplitude imbalance between the paths of the output signals341A—341D. Some signal processing schemes may be used to correct for theimperfections of the 90° optical hybrid receiver, and to account for theeffects of up/down chirped signals from the optical source.

As depicted in FIG. 7A, there may be imperfections in the 90° opticalhybrid 130 itself, such as the angle not being exactly 90°. The hybridangle 701 between the output mixed signals I and Q may have the value“x”. There may be amplitude imbalances between the I and Q paths, oreven physical delay differences in the I and Q paths from the 90°optical hybrid 130, or even after the 90° optical hybrid 130 up throughthe signal processing chain. As a result of the optical hybrid angle 701and/or the amplitude imbalance, the negative peak 703 b may not becompletely suppressed, as depicted in FIG. 7B. The image rejection ratio(IRR) may be determined by the optical hybrid angle 701 and theamplitude imbalance. As an example, the IRR may refer to a signalstrength of the positive image peak 703 a to that of the negative imagepeak 703 b of the reference beat signal 703. The IRR is usuallyexpressed in dB.

The imperfections of the 90° optical hybrid 130 may be mitigated in theDSP 117. In one embodiment, the imperfections of the 90° optical hybrid130 may be corrected by digitally adjusting the amplitude of the signalI and/or Q after normalization. For example, at least one of theamplitude of the signal I or Q may be digitally adjusted afternormalization.

In one embodiment, the imperfections of the 90° optical hybrid 130 maybe corrected by combining I and Q at an angle slightly different from90°. For example, combining the signal I and Q at 90° may be expressedas: I+jQ, where j=e(j90π/180); while combining the signal I and Q at anangle x off from 90° may be expressed I+e(j(90−x)π/180)Q.

In one embodiment, the imperfections of the 90° optical hybrid 130 maybe corrected by de-skewing the signal I and/or Q in time to mitigate anytime delay between them. For example, at least one of the signal Iand/or Q may be de-skewed in time to mitigate any time delay betweenthem. The optimal values for these adjustments may be done as a simplecalibration and the image rejection ratio between the positive andnegative image of the reference beat frequency may be used as thecalibration feedback.

FIG. 8 is a flow diagram illustrating an example of a method 800 ofcorrecting for imperfections of a 90° optical hybrid receiver accordingto embodiments of the present disclosure. Method 800 may be performed byprocessing logic that may comprise hardware (e.g., circuitry, dedicatedlogic, programmable logic, a processor, a processing device, a centralprocessing unit (CPU), a system-on-chip (SoC), etc.), software (e.g.,instructions running/executing on a processing device), firmware (e.g.,microcode), or a combination thereof. For example, the method 800 may beperformed by a processor, e.g., a signal processing unit 112 of a LiDARsystem, as illustrated in FIG. 1A-FIG. 1B. In this method, theimperfections of the 90° optical hybrid 130 may be corrected bydigitally adjusting the amplitude of I and Q after normalization,combining I and Q at an angle slightly different from 90°, or de-skewingI and Q signals in time to mitigate any time delay between them.

Referring to FIG. 8 , at block 801, a calibration of the combinedreference signal is performed. As an example, the incoming mixed signals326 and 328 (e.g., I and Q) data from the ADCs 325, 327 may benormalized. For example, a feedback of the calibration comprises an IRRbetween a positive image and the negative image of the reference beatfrequency.

At block 802, the amplitude of the incoming mixed signals 326 and 328are digitally adjust after normalization. As an example, the amplitudeof the incoming mixed signals 326 and 328 are digitally adjust tocompensate for the amplitude imbalance.

At block 804, the incoming mixed signals 326 and 328 are combined at anangle different from 90°. As an example, the amplitude of the incomingmixed signals 326 and 328 are combined to compensate for the hybridangle x.

At block 806, the incoming mixed signals 326 and 328 are de-skewed in atime domain to mitigate any time delay between the mixed signal 326 andthe mixed signal 328.

By this way, the IRR between the positive image and the negative imageof the reference beat frequency is increased, thereby improving thetarget SNR and the accuracy of the velocity and range estimation of thetarget.

FIG. 9 is a flow diagram illustrating an example of a method 900 ofusing a coherent reference receiver in a LiDAR system according toembodiments of the present disclosure. Method 900 may be performed byprocessing logic that may comprise hardware (e.g., circuitry, dedicatedlogic, programmable logic, a processor, a processing device, a centralprocessing unit (CPU), a system-on-chip (SoC), etc.), software (e.g.,instructions running/executing on a processing device), firmware (e.g.,microcode), or a combination thereof. For example, the method 900 may beperformed by a processor, e.g., a signal processing unit 112 of a LiDARsystem, as illustrated in FIG. 1A-FIG. 1B. By this method, the negativeimage of the reference beat frequency is suppressed, and the target SNRis improved. Therefore, the accuracy of determining the range andvelocity information of the target is improved.

Referring to FIG. 9 , at block 902, an optical beam is emitted by anoptical source along a target path towards a target and a referencepath.

At block 904, a portion of the optical beam and a LO signal are receivedand mixed by a coherent receiver disposed in the reference path. Block904 includes blocks 906, 908 and 910. At block 906, the portion of theoptical beam and the LO signal are received by a 90° optical hybrid togenerate a first, a second, a third and a fourth output signal. At block908, the first and the second output signal are received by a firstphotodetector to generate a first mixed signal. At block 910, the thirdand the fourth output signal are received by a second photodetector togenerate a second mixed signal.

At block 912, combining the first mixed signal and the second mixedsignal are combined by a processor to generate a combined referencesignal to suppress a negative image of a beat frequency signal producedby the optical beam and the LO signal to estimate a phase noise of theoptical source to determine range and velocity information of thetarget.

The preceding description sets forth numerous specific details such asexamples of specific systems, components, methods, and so forth, inorder to provide a thorough understanding of several examples in thepresent disclosure. It will be apparent to one skilled in the art,however, that at least some examples of the present disclosure may bepracticed without these specific details. In other instances, well-knowncomponents or methods are not described in detail or are presented inblock diagram form in order to avoid unnecessarily obscuring the presentdisclosure. Thus, the specific details set forth are merely exemplary.Particular examples may vary from these exemplary details and still becontemplated to be within the scope of the present disclosure.

Any reference throughout this specification to “one example” or “anexample” means that a particular feature, structure, or characteristicdescribed in connection with the examples are included in at least oneexample. Therefore, the appearances of the phrase “in one example” or“in an example” in various places throughout this specification are notnecessarily all referring to the same example.

Although the operations of the methods herein are shown and described ina particular order, the order of the operations of each method may bealtered so that certain operations may be performed in an inverse orderor so that certain operations may be performed, at least in part,concurrently with other operations. Instructions or sub-operations ofdistinct operations may be performed in an intermittent or alternatingmanner.

The above description of illustrated implementations of the invention,including what is described in the Abstract, is not intended to beexhaustive or to limit the invention to the precise forms disclosed.While specific implementations of, and examples for, the invention aredescribed herein for illustrative purposes, various equivalentmodifications are possible within the scope of the invention, as thoseskilled in the relevant art will recognize. The words “example” or“exemplary” are used herein to mean serving as an example, instance, orillustration. Any aspect or design described herein as “example” or“exemplary” is not necessarily to be construed as preferred oradvantageous over other aspects or designs. Rather, use of the words“example” or “exemplary” is intended to present concepts in a concretefashion. As used in this application, the term “or” is intended to meanan inclusive “or” rather than an exclusive “or”. That is, unlessspecified otherwise, or clear from context, “X includes A or B” isintended to mean any of the natural inclusive permutations. That is, ifX includes A; X includes B; or X includes both A and B, then “X includesA or B” is satisfied under any of the foregoing instances. In addition,the articles “a” and “an” as used in this application and the appendedclaims should generally be construed to mean “one or more” unlessspecified otherwise or clear from context to be directed to a singularform. Furthermore, the terms “first,” “second,” “third,” “fourth,” etc.as used herein are meant as labels to distinguish among differentelements and may not necessarily have an ordinal meaning according totheir numerical designation.

What is claimed is:
 1. A light detection and ranging (LiDAR) system,comprising: an optical source to emit an optical beam along a targetpath towards a target and a reference path; a coherent receiver disposedin the reference path, comprising: a 90° optical hybrid to receive aportion of the optical beam along the reference path and a localoscillator (LO) signal to generate a first, a second, a third and afourth output signal; a first photodetector to receive the first and thesecond output signal to generate a first mixed signal; and a secondphotodetector to receive the third and the fourth output signal togenerate a second mixed signal, wherein the coherent receiver isdisposed to mix the portion of the optical beam with the LO signal; anda processor to combine the first mixed signal and the second mixedsignal to generate a combined reference signal to suppress a negativeimage of a reference beat frequency signal produced by the optical beamand the LO signal to estimate a phase noise of the optical source todetermine range and velocity information of the target, wherein theprocessor is further configured to improve an image rejection ratio tocorrect a hybrid angle and amplitude balance imperfections to suppressthe negative image.
 2. The LiDAR system of claim 1, wherein theprocessor is further configured to compare a target return signal fromthe target with the combined reference signal to estimate the phasenoise.
 3. A light detection and ranging (LiDAR) system, comprising: anoptical source to emit an optical beam along a target path towards atarget and a reference path; a coherent receiver disposed in thereference path, comprising: a 90° optical hybrid to receive a portion ofthe optical beam along the reference path and a local oscillator (LO)signal to generate a first, a second, a third and a fourth outputsignal; a first photodetector to receive the first and the second outputsignal to generate a first mixed signal; and a second photodetector toreceive the third and the fourth output signal to generate a secondmixed signal, wherein the coherent receiver is disposed to mix theportion of the optical beam with the LO signal; and a processor tocombine the first mixed signal and the second mixed signal to generate acombined reference signal to suppress a negative image of a referencebeat frequency signal produced by the optical beam and the LO signal toestimate a phase noise of the optical source to determine range andvelocity information of the target, wherein the processor is to generatethe combined reference signal by applying a polarity correction.
 4. TheLiDAR system of claim 3, wherein the processor is to determine a sign ofthe second mixed signal depending on a direction of frequency sweeping.5. The LiDAR system of claim 4, wherein, provided the direction is anup-sweeping direction, the processor is to determine the sign of thesecond mixed signal to be a first sign.
 6. The LiDAR system of claim 5,wherein, provided the direction is a down-sweeping direction, theprocessor is to determine the sign of the second mixed signal to be asecond sign which is an opposite sign of the first sign.
 7. The LiDARsystem of claim 4, wherein the processor is to apply a square wave tochange the sign.
 8. The LiDAR system of claim 1, wherein processor isfurther configured to improve an image rejection ratio by performing acalibration of the combined reference signal.
 9. The LiDAR system ofclaim 8, wherein a feedback of the calibration comprises an imagerejection ratio between a positive image and the negative image of thereference beat frequency.
 10. The LiDAR system of claim 8, wherein theprocessor is to digitally adjust an amplitude of at least one of thefirst mixed signal or the second mixed signal after normalization. 11.The LiDAR system of claim 8, wherein the processor is to combine thefirst mixed signal and the second mixed signal at an angle differentfrom 90°.
 12. The LiDAR system of claim 8, wherein the processor is tode-skew at least one of the first mixed signal or the second mixedsignal in a time domain to mitigate any time delay between the firstmixed signal and the second mixed signal.
 13. A method of lightdetection and ranging (LiDAR), comprising: emitting an optical beam byan optical source along a target path towards a target and a referencepath; receiving and mixing a portion of the optical beam and a localoscillator (LO) signal by a coherent receiver disposed in the referencepath, comprising: receiving the portion of the optical beam and the LOsignal by a 90° optical hybrid to generate a first, a second, a thirdand a fourth output signal; receiving the first and the second outputsignal by a first photodetector to generate a first mixed signal; andreceiving the third and the fourth output signal by a secondphotodetector to generate a second mixed signal; combining, by aprocessor, the first mixed signal and the second mixed signal togenerate a combined reference signal to suppress a negative image of areference beat frequency signal produced by the optical beam and the LOsignal to estimate a phase noise of the optical source to determinerange and velocity information of the target; and improving, by theprocessor, an image rejection ratio to correct a hybrid angle andamplitude balance imperfections to suppress the negative image.
 14. Themethod of claim 13, further comprising comparing, by the processor, atarget return signal from the target with the combined reference signalto estimate the phase noise.
 15. A method of light detection and ranging(LiDAR), comprising: emitting an optical beam by an optical source alonga target path towards a target and a reference path; receiving andmixing a portion of the optical beam and a local oscillator (LO) signalby a coherent receiver disposed in the reference path, comprising:receiving the portion of the optical beam and the LO signal by a 90°optical hybrid to generate a first, a second, a third and a fourthoutput signal; receiving the first and the second output signal by afirst photodetector to generate a first mixed signal; and receiving thethird and the fourth output signal by a second photodetector to generatea second mixed signal; and combining, by a processor, the first mixedsignal and the second mixed signal to generate a combined referencesignal to suppress a negative image of a reference beat frequency signalproduced by the optical beam and the LO signal to estimate a phase noiseof the optical source to determine range and velocity information of thetarget, wherein the combining, by the processor, the first mixed signaland the second mixed signal to generate the combined reference signalcomprises combining, by the processor, the first mixed signal and thesecond mixed signal to generate the combined reference signal byapplying a polarity correction.
 16. The method of claim 15, wherein theapplying the polarity correction comprises determining a sign of thesecond mixed signal depending on a direction of frequency sweeping. 17.The method of claim 16, wherein the applying the polarity correctioncomprises, provided the direction is an up-sweeping direction,determining the sign of the second mixed signal to be a first sign. 18.The method of claim 16, wherein the applying the polarity correctioncomprises, provided the direction is a down-sweeping direction,determining the sign of the second mixed signal to be a second signwhich is an opposite sign of the first sign.
 19. The method of claim 13,wherein the combining, by the processor, the first mixed signal and thesecond mixed signal to generate the combined reference signal comprisesperforming a calibration of the combined reference signal.